1. Field of the Invention
The invention relates to a control device for an internal combustion engine, a vehicle and a control method for an internal combustion engine and, more particularly, to control for setting the idle rotation speed of an internal combustion engine.
2. Description of Related Art
In an internal combustion engine, such as an engine, the rotation speed of the engine in so-called idle operation in which the engine is autonomously operated in a state where driving force is not transmitted to a load after the start of the engine (hereinafter, also referred to as “idle rotation speed”) is desirably set so as to be as low as possible within the range in which autonomous operation is possible in order to reduce fuel consumption.
On the other hand, while the engine is being operated, vibrations occur because of the operation of the engine; however, in order to reduce vibrations during idle operation, the idle rotation speed is set so as to be higher than a rotation speed at which resonance is caused to occur in a driving force transmission system including the engine (hereinafter, also referred to as “resonance rotation speed”).
Japanese Patent Application Publication No. 2006-152877 (JP 2006-152877 A) describes a configuration that, in a hybrid vehicle that cranks a mounted engine with a motor to start the engine, when an increase in the rotation speed of the engine is suppressed at the time of cranking the engine and, therefore, the rotation speed of the engine at the time of cranking may coincide with the resonance rotation speed of the driving force transmission system, the motor is driven such that the rotation speed of the engine is lower than the resonance rotation speed.
With the configuration described in JP 2006-152877 A, at the time of cranking the engine to start the engine, even when the rotation speed of the engine may coincide with the resonance rotation speed because of an increase in friction torque, a decrease in motor output due to a decrease in battery output, or the like, it is possible to suppress the resonance of the driving force transmission system.
Generally, in order to reduce vibrations during idle operation, the idle rotation speed of the engine is set to a value different from a rotation speed corresponding to the resonance frequency of the driving force transmission system to which vibrations are transmitted from the engine (resonance rotation speed).
However, for example, when the vehicle has been continuously maintained in a state where the engine is stopped for a long period of time in a low-temperature environment (for example, −15° C. or below) in a cold place, or the like, the resonance rotation speed of the driving force transmission system may vary. Therefore, when the vehicle has been continuously maintained in a state where the engine is stopped in a low-temperature environment, vibrations during idle rotation may increase because the resonance rotation speed of the driving force transmission system approaches the idle rotation speed.
On the other hand, in recent years, for the purpose of improvement in fuel economy, there are vehicles that have an idle stop function or an economy running function that automatically stops the engine when the vehicle is stopped to wait at a traffic light, for example. In addition, in hybrid vehicles on which an engine and an electric motor are mounted, some of the hybrid vehicles are able to carry out an electric vehicle (EV) running mode in which only driving force from the electric motor is used to run while the engine is stopped, and, in such hybrid vehicles as well, the engine may be automatically stopped during the travel of the vehicle.
In this way, in a vehicle that has the function of automatically stopping the engine during the travel of the vehicle, in the case where the vehicle is left standing for a long period of time in a low-temperature environment as described above, when the engine is frequently stopped, energy, such as heat and vibrations, from the engine is not transmitted to the driving force transmission system, and such vibrations that occur at the time of the engine start may continuously occur for a long period of time.